The present invention concerns a process and apparatus for treating a gaseous effluent from a Claus plant or a gas containing hydrogen sulphide and sulphur dioxide.
In particular, it concerns the treatment of effluents from Claus plants particularly from hydrodesulphurization and catalytic cracking units. It also concerns the purification treatment of natural gas.
The prior art is illustrated by French patent applications FR-A-2 411 802 and FR-A-2 336 163.
The Claus process is widely used to recover elemental sulphur from gaseous feeds containing hydrogen sulphide (H.sub.2 S). However, the fumes emitted from these Claus type plants, even after several catalytic stages, contain non-negligible amounts of acid gases. Those effluents (tail gases) from Claus plants must, therefore, be treated to eliminate the majority of toxic compounds so as to satisfy anti-pollution regulations. These regulations are becoming more and more strict and existing technology must be constantly improved.
As an example, about 95% by weight of the sulphur present can be recovered from a Claus plant; treatment of this Claus plant tail gas (using a Clauspol unit, for example) can recover 99.8% by weight of the sulphur, for example, using the reaction: EQU 2H.sub.2 S+SO.sub.2 .fwdarw.3S+2H.sub.2 O
which uses a reaction medium constituted by an organic solvent and a catalyst comprising an alkaline or alkaline-earth salt of an organic acid. The reaction is generally carried out in counter-current mode in a reactor-contactor and its temperature is controlled by passing the solvent which is extracted from the lower end of the reactor by a circulating pump through a heat exchanger to encourage the highest possible degree of conversion to sulphur while avoiding the formation of solid sulphur. Sulphur is thus recovered in liquid form. While it is very effective, the process is limited by various constraints:
The thermodynamic equilibrium of the reaction is such that the reaction is never complete. Some hydrogen sulphide and sulphur dioxide remains, in equilibrium with the sulphur and water which are formed. The quantity of sulphur present in unreacted H.sub.2 S and SO.sub.2 which is found in the reaction effluent (from the Clauspol) corresponds to about 0.1% of the total sulphur in the initial feed to the Claus plant. Better conversion can be envisaged at a lower operating temperature but this temperature must be kept above the freezing point of sulphur (about 120.degree. C.) otherwise the reactor will be blocked with solid sulphur; PA1 The presence of unseparated liquid sulphur in the reactor-contactor, which is entrained in the solvent and catalyst which circulate, and which is recycled to the reactor-contactor. Not all of the droplets of liquid sulphur are separated from the solvent and the presence of liquid sulphur irremediably causes the presence of gaseous sulphur in the effluent due to the vapor pressure of sulphur. As an example, the quantity of unrecovered sulphur which can be attributed to vapor pressure is about 0.1% by weight of the sulphur in the initial feed. PA1 In a first variation, if the organic solvent is miscible with water, it can be cooled by heat exchange in a heat exchanger before being mixed with the gaseous effluent to be purified, by adding water at a temperature which is lower than that of the organic solvent, wherein the heat of vaporization on contact with the gaseous effluent can reduce the temperature of the mixture, or by a combination of the above two steps. Cooling is preferably by injection of water. PA1 In a second variation, if the organic solvent is not miscible with water, it can be cooled in the same manner as in the first variation. Cooling is preferably by heat exchange. PA1 In the category of solvents which are insoluble in water are hydrocarbons with boiling points at atmospheric pressure of more than 200.degree. C., preferably dodecane, tridecane, and naphtha with boiling points in the range 225.degree. C. to 335.degree. C. PA1 In the category of solvents which are soluble in water, with boiling points at atmospheric pressure of more than 200.degree. C. are polyols containing 3 to 15 carbon atoms, preferably glycerol, thioglycol and cyclohexanedimethylethanol, acid esters containing 5 to 15 carbon atoms, more particularly trimethylpentanediol mono-isobutyrate and dimethyl adipate, glycol ethers containing 5 to 15 carbon atoms, advantageously butoxytriglycol, ethoxytriglycol, diethylene glycol butylether, ethylene glycol phenylether, terpinyl ethylene glycol monobenzyl ether, ethylene glycol butylphenylether, diethylene glycol, diethylene glycol dimethylether, diethylene glycol dibutylether, triethylene glycol, tetraethylene glycol dimethylether, propylene n-butylether, dipropylene n-butylether, tripropylene n-butylether, and polyethylene glycol with a molar mass of 200, 300, 400 or 600. PA1 The gas to be treated is a co-current with the solvent. In this case, the gas to be treated is introduced to the head of the reactor-contactor along with the recycled solvent originating from a side stream extracted from the lower portion of the reactor. The gaseous effluent is also extracted as a side stream from the lower portion of the reactor, above the solvent to be recycled. PA1 The gas to be treated is a counter-current to the solvent. In this case, the gas to be treated is introduced as a side stream to the lower portion of the reactor-contactor, and the solvent originating from the lower portion of the reactor is recycled to the upper portion as a side stream. Sulphur is recovered from the bottom of the reactor and the gaseous effluent for cooling is extracted overhead.
The aim of the invention is to overcome the disadvantages of the prior art.
A further aim of the invention is to satisfy the strictest regulations designed to counter atmospheric pollution by sulphur-containing compounds.
A yet still further aim of the invention is to be able to modify existing installations with a Claus plant and a unit for treating the effluents from that unit (a Clauspol unit), at very low cost.
It has been shown that by eliminating all of the sulphur vapor from the effluents from gas treatment units, for example Claus plant tail gas, up to 99.9% of the total sulphur can be recovered and thus the quantity of sulphur discharged into the atmosphere when the gas is incinerated can be minimised.